Image display apparatus

ABSTRACT

An image display apparatus using a display panel such as a plasma display panel having two illumination states aims to widen a dynamic range by increasing a ratio of maximum to minimum luminance reproducible on the same screen. In the image display apparatus, one TV field period is divided into a plurality of sub-fields which respectively have luminance weights and are arranged in order of time, and a gray-scale image is displayed by selecting a combination of sub-fields for each pixel and sustaining a light emission state of each pixel during the selected sub-fields, wherein when arranged in ascending order of luminance weight, the plurality of sub-fields include at least one sub-field whose luminance weight is one-half of a luminance weight of the next sub-field.

TECHNICAL FIELD

The present invention relates to an image display apparatus which uses adisplay panel such as a plasma display panel that divides one TV fieldperiod into a plurality of sub-fields to display a gray-scale image, thedisplay apparatus being capable of displaying an image of a wide dynamicrange by increasing a ratio of maximum to minimum luminance reproducibleon the same screen.

The invention further relates to an image display apparatus that dividesone TV field period into a plurality of sub-fields to display agray-scale image, the display apparatus being capable of reducinghalftone disturbances which occur when displaying a moving image.

BACKGROUND ART

To display a gray-scale image on a display panel such as a plasmadisplay panel that is fundamentally only capable of two display states,a method is widely employed that separates one TV field period intosub-fields, assigns predetermined luminance weights to the sub-fields,and controls the presence or absense of light emission of eachsub-field.

For instance, 256 levels of gray are represented by dividing a TV fieldperiod into eight sub-fields that are respectively given the luminanceweights of “1”, “2”, “4”, “8”, “16”, “32”, “64”, and “128”. When aninput image signal is an 8-bit digital signal, then the 8 bits arerespectively assigned to the eight sub-fields starting with the leastsignificant bit. Here, each sub-field image has two display states.

A CRT display bears the so-called inverse gamma property, so that whilemaximum luminance is proportional to “255”, minimum luminance isproportional to a decimal no greater than “1”. Hence the dynamic rangeis kept at a sufficient level of 255 or higher.

On the other hand, a plasma display panel has a linear luminousproperty, so that a gray level is expressed by a sum of luminance levelssubstantially proportional to sub-field weights. Which is to say, whilemaximum luminance is proportional to a sum of luminance weights of allsub-fields, i.e. “255”, minimum luminance is proportional to “1”.Because of this greater minimum luminance than that of the CRT, thedynamic range of the plasma display panel is narrower than the CRT.

The dynamic range of the plasma display panel may be widened byincreasing the number of sub-fields so as to increase the number ofreproducible levels of gray, but this technique is not easy to implementdue to restrictuions such as discharge speeds of plasma display panels.Therefore, the number of sub-fields is normally limited.

Also, the aforementioned method of expressing 256 levels of gray usingthe eight sub-fields is known to be susceptible to halftone disturbanceswith significant false contours which appear when displaying a movingimage.

To reduce such halftone disturbances, a technique has been devised thatdetects motion in an image and switches coding for each pixel or eachimage portion in the image.

As an example of this technique, coding is varied for each image portionsuch that when input is made in 256-level gray scale, light emission iseffected in 256 levels of gray for a static image portion, while lightemission is effected in a more limited number of gray levels for amoving image portion. In so doing, the moving image portion is coded sothat the light-emission pattern changes with a certain degree ofcontinuity against monotonous gray level changes of input image signals.This benefits a reduction of annoying false contours in the moving imagedisplay. Meanwhile, a desired sufficient gray scale is guaranteed in thestatic image display.

In such a conventional method, however, coding is switched at theboundary of the moving and static portions. In some images, thisswitching causes a certain impact on the boundary area. The impact ofthe switching is particularly well observed in boundaries of an objectthat is moving in plane within an image.

DISCLOSURE OF INVENTION

To solve the stated problems, the first object of the invention is toprovide an image display apparatus equipped with a plasma display panelor the like, that divides one TV field period into a plurality ofsub-fields to produce a gray-scale image, and that can display an imagewith a truly wide dynamic range by increasing a ratio between maximumand minimum luminance reproducible on the same screen.

The second object of the invention is to provide a gray-scale imagedisplay apparatus that not only reduces halftone disturbances whichappear when displaying a moving image, but lessens an impact ofswitching between different coding modes.

The first object can be fulfilled by an image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by selecting a combination of sub-fields foreach pixel and sustaining a light emission state in each pixel duringthe selected sub-fields, characterized in that when arranged inascending order of luminance weight, the plurality of sub-fields includeat least one sub-field whose luminance weight is smaller than one-halfof a luminance weight of the next sub-field.

The first object can also be fulfilled by an image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by selecting a combination of sub-fields foreach pixel and sustaining a light emission state in each pixel duringthe selected sub-fields, characterized in that when the plurality ofsub-fields are arranged in ascending order of luminance weight with an“i”th smallest luminance weight being denoted by W_(i), the plurality ofsub-fields are respectively given such luminance weights that “n” existswhere W₁+W₁+W₂+ . . . +W_(n)<W_(n+1).

With this construction, when all reproducible luminance levels (graylevels) are rearranged in ascending order of luminance level (graylevel), the luminance level (gray level) jumps by one or more levels atcertain points. This makes it possible to increase the ratio betweenminimum to maximum luminance reproducible on the same screen, incomparison with the conventional techniques. As a result, an imagedisplay of a wide dynamic range can be realized.

The first object can also be fulfilled by an image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by selecting a combination of sub-fields foreach pixel and sustaining a light emission state in each pixel duringthe selected sub-fields, characterized in that when the plurality ofsub-fields are arranged in ascending order of luminance weight with a“j”th smallest luminance weight being denoted by W_(j), the plurality ofsub-fields are respectively given such luminance weights that “n” and atleast two “i”s exist where W_(i)+W₁+W₂+ . . . +W_(n)<W_(n+1).

With this construction, when all reproducible luminance levels (graylevels) are rearranged in ascending order of luminance level (graylevel), the luminance level (gray level) jumps by one or more levels atcertain points. This makes it possible to increase the ratio betweenminimum to maximum luminance reproducible on the same screen, incomparison with the conventional techniques. As a result, an imagedisplay of a wide dynamic range can be realized. Furthermore, theamounts of jumps in luminance level can be controlled in accordance withgray levels of input image signals. For instance, the higher the inputgray level, the luminance level is made to jump by a greater amount.This further increases the reproducible maximum luminance.

The first object can also be fulfilled by an image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by selecting a combination of sub-fields foreach pixel and sustaining a light emission state in each pixel duringthe selected sub-fields, wherein a coding pattern that specifies a sumof luminance weights of all sub-fields in the current TV field period isdetermined in accordance with a characteristic of input pixel imagesignals corresponding to the image of the current TV field period,characterized in that when a reference TV field period is divided into aplurality of sub-fields that are respectively given luminance weights,and a ratio of the sum of luminance weights of all sub-fields in thecurrent TV field period to a sum of luminance weights of all sub-fieldsin the reference TV field period is denoted by K, the current TV fieldperiod includes (a) one or more sub-fields whose luminance weights areobtained by multiplying luminance weights of predetermined sub-fields inthe reference TV field period, respectively by coefficients no greaterthan K, and (b) one or more sub-fields whose luminance weights areobtained by multiplying luminance weights of predetermined sub-fields inthe reference TV field period, respectively by coefficients greater thanK.

With this construction, the reproducible minimum luminance is kept low,while the reproducible maximum luminance is controlled in accordancewith a distribution of gray levels in an image. In general, when animage contains an area of relatively high brightness, if thereproducible maximum luminance is raised higher than necessary, there isa danger that the total power consumption may increase in a displaydevice such as a plasma display panel where power consumption closelycorrelates with reproduced luminance. Therefore, it is desirable tocontrol the reproducible maximum luminance depending on thecharacteristic of the image. To be more specific, low luminance weightsof sub-fields are always kept relatively low, whereas high luminanceweights of sub-fields are changed according to a desired maximumluminance level. Accordingly, the ratio of maximum to minimum luminanceis increased. Also, even if the maximum luminance is reproduced at ahigh level, the corresponding image area will not be isolated within theimage, and good contrast will not be impaired.

Here, the coefficients no greater than K and the coefficients greaterthan K may be determined based on a rule which is defined by anascending order of luminance weight in the reference TV field period.

Here, the coefficients determined based on the rule may be coefficientsthat monotonously increase in ascending order of luminance weight in thereference TV field period.

Here, the coefficients determined based on the rule may be coefficientsthat increase in arithmetic progression in ascending order of luminanceweight in the reference TV field period.

Here, the coefficients determined based on the rule may be coefficientsthat increase in geometric progression in ascending order of luminanceweight in the reference TV field period.

Here, the sub-fields whose luminance weights are obtained by themultiplications by the coefficients no greater than K may include asub-field whose luminance weight is obtained by a multiplication by acoefficient within a range that is fixed irrespective of which value Ktakes.

Here, in each of at least two coding patterns among a plurality ofcoding patterns from which the coding pattern of the current TV fieldperiod is selected, at least two sets of three luminance weightsselected in ascending order of luminance weight may each meet thecondition that the three luminance weights approximately have aproportion selected from the group consisting of “1:2:3”, “1:2:4”,“1:2:5”, “1:2:6”, “1:3:7”, “1:4:9” “2:6:12”, and “2:6:16”.

Here, when S denotes the sum of luminance weights of all sub-fields inthe current TV field period and R is within a range from 0 to S, a graylevel corresponding to R may be expressed by selecting a combination ofsub-fields whose luminance weights, when added together, are closest toR.

With this construction, a gray level which cannot be expressed with asingle combination of sub-fields can be corrected using a known graylevel correction technique such as error diffusion or dithering.Accordingly, the minimum luminance is kept low, whereas the reproduciblemaximum luminance is made high, with it being possible to produce anexcellent image display of a wide dynamic range with corrected, smoothgray levels.

Here, the selection of the combination of sub-fields for each pixel maybe controlled in accordance with one out of: an amount of movement froman image of a past TV field period to the image of the current TV fieldperiod; and an approximate value of the amount of movement.

With this construction, the minimum luminance is kept low, whereas thereproducible maximum luminance is made high, with it being possible toproduce an excellent image display of a wide dynamic range withcorrected, smooth gray levels. Moreover, the occurrence of falsecontours in a moving image can be suppressed.

Note that moving image false contours occur when a viewer's eyes moverelative to a subject within an image. Still, false contours can besubstantially suppressed by using an amount of movement in an image oran approximate value of the amount of movement.

Here, in an image area where the amount of movement or the approximatevalue of the amount of movement is large, such combinations ofsub-fields may be selected that monotonously increase in time withincreasing gray levels of input pixel image signals.

With this construction, when the input gray level rises, no sub-field isswitched from the ON to the OFF state, or only sub-fields withrelatively small luminance weights are switched from the ON to the OFFstate. In so doing, the occurrence of moving image false contours issuppressed more effectively.

The second object can be fulfilled by an image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes that are switched in accordance with an amount ofmovement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each pixel depending on the amount of movement, and a light emissionstate is sustained in each pixel during the selected sub-fields,characterized in that the different coding modes are interspersedlyapplied to input pixel image signals that correspond to an image areawhere switching between the different coding modes is needed and thatshow a predetermined characteristic.

The second object can also be fulfilled by an image display apparatus,in which a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes that are switched in accordance with an amount ofmovement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each pixel depending on the amount of movement, and a light emissionstate is sustained in each pixel during the selected sub-fields,characterized in that a signal used for switching between the differentcoding modes is arbitrarily space-modulated so that the different codingmodes are interspersedly applied to input pixel image signals thatcorrespond to an image area where the switching between the differentcoding modes is needed and that show a predetermined characteristic.

The second object can also be fulfilled by an image display apparatus,in which a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes that are switched in accordance with an amount ofmovement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each pixel depending on the amount of movement, and a light emissionstate is sustained in each pixel during the selected sub-fields,characterized in that a signal used for switching between the differentcoding modes is regularly space-modulated so that the different codingmodes are interspersedly applied to input pixel image signals thatcorrespond to an image area where the switching between the differentcoding modes is needed and that show a predetermined characteristic.

The second object can also be fulfilled by an image display apparatus,in which a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes which are switched in accordance with an amountof movement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each pixel depending on the amount of movement, and a light emissionstate is sustained in each pixel during the selected sub-fields,characterized in that a signal used for switching between the differentcoding modes, when expressed pixel by pixel as a virtual image of amatrix form in a plane, assumes a shape that contains a zigzag as a maincomponent which turns no more than once in a pixel, so that thedifferent coding modes are interspersedly applied to input pixel imagesignals that correspond to an image area where the switching between thedifferent coding modes is needed and that show a predeterminedcharacteristic.

With these constructions, the switching between the different codingmodes is performed gradually, so that an impact of the switching isalleviated while suppressing moving image false contours. This benefitssmooth switching between different coding modes such as static imagecoding and moving image coding.

Here, the shape that contains the zigzag as the main component may havea pattern in which adjacent pixels alternate between two states.

Here, the shape that contains the zigzag as the main component may be ashape that randomly combines zigzags each of which turns no more thanonce in a pixel.

Here, the input pixel image signals that show the predeterminedcharacteristic may correspond to a non-edge image area.

With this construction, an impact of the switching between the differentcoding modes is suppressed in the non-edge image area that isparticularly susceptible to such an impact, while the switching of thedifferent coding modes is swiftly performed in the edge image area.Hence coding that is suitable for each image area is accomplishedwithout decreasing the average signal-to-noise ratio of the whole image.

The second object can also be fulfilled by an image display apparatus,in which a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes which are switched in accordance with an amountof movement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each. pixel depending on the amount of movement, and a lightemission state is sustained in each pixel during the selectedsub-fields, characterized in that a modulation signal having periodicitycorresponding to no smaller than a pixel interval is applied to inputpixel image signals that correspond to an image area where switchingbetween the different coding modes is needed.

The second object can further be fulfilled by an image displayapparatus, in which a current TV field period is divided into aplurality of sub-fields that are respectively given luminance weightsand are arranged in order of time, and a gray-scale image for thecurrent TV field period is displayed by coding input pixel image signalsusing different coding modes which are switched in accordance with anamount of movement from an image of a past TV field period to the imageof the current TV field period, wherein a combination of sub-fields isselected for each pixel depending on the amount of movement, and a lightemission state is sustained in each pixel during the selectedsub-fields, characterized in that input pixel image signalscorresponding to an image area where switching between the differentcoding modes is needed are modulated to shift a display position of theimage area.

With these constructions, the switching between the different codingmodes is performed gradually, so that an impact of the switching isalleviated while suppressing moving image false contours. This benefitssmooth switching between different coding modes such as static imagecoding and moving image coding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a construction of an image displayapparatus according to the first embodiment of the invention;

FIG. 2 shows the correspondence between input image signal values andconverted image signal values in a static image coding circuit;

FIG. 3 shows the correspondence between input image signal values andconverted image signal values in a moving image coding circuit;

FIG. 4 is a block diagram showing a construction of a motion detectioncircuit;

FIG. 5 is a block diagram showing a construction of a sub-field controlcircuit;

FIG. 6 shows the correspondence between input image signal values andfield information;

FIG. 7 shows constructions of frame memories in the sub-field controlcircuit;

FIG. 8 is a block diagram showing a construction of a display controlcircuit;

FIG. 9 shows a PDP drive method;

FIGS. 10(a)-10(c) are diagrams showing the correlation between inputimage signal values and reproduced luminance levels;

FIG. 11 is a block diagram showing a construction of an image displayapparatus according to the second embodiment of the invention;

FIGS. 12(a)-12(e) are diagrams showing a process of switching codingpatterns based on a value of K in the sub-field control circuit (priorart);

FIGS. 13(a)-13(e) are characteristic diagrams showing the correlationbetween input image signal values and reproduced luminance levels (priorart);

FIGS. 14(a)-14(e) are diagrams showing a process of switching codingpatterns based on a value of K in the sub-field control circuit (presentinvention);

FIGS. 15(a)-15(e) are characteristic diagrams showing the correlationbetween input image signal values and reproduced luminance levels(present invention);

FIG. 16 is a block diagram showing a construction of an image displayapparatus according to the third embodiment of the invention;

FIG. 17 shows, by way of example, an input image and a motion detectionresult;

FIG. 18 is a block diagram showing a construction of an image displayapparatus according to the fourth embodiment of the invention;

FIG. 19 shows, by way of example, an input image and a motion detectionresult;

FIG. 20 is a block diagram showing a construction of an image displayapparatus according to the fifth embodiment of the invention;

FIG. 21 shows coding modes of respective image coding circuits;

FIG. 22 is a block diagram showing a construction of an image displayapparatus according to the sixth embodiment of the invention;

FIG. 23 shows, by way of example, an input image and a motion detectionresult;

FIG. 24 is a block diagram showing a construction of an image displayapparatus according to the seventh embodiment of the invention;

FIG. 25 shows, by way of example, an input image and a motion detectionresult; and

FIGS. 26(a)-26(c) are characteristic diagrams showing the coding patternand the correlation between input image signal values and reproducedluminance levels when K=2.5.

BEST MODE FOR CARRYING OUT THE INVENTION

The following is a description of an image display apparatus accordingto embodiments of the invention, with reference to the figures.

First Embodiment

(General Construction)

An image display apparatus of the first embodiment uses an AC-typeplasma display panel (hereinafter, “PDP”). This image display apparatusproduces a halftone image by expressing a gray scale with a sum of lightemissions for a predetermined number of sub-fields (e.g. ten sub-fields)that are respectively assigned predetermined numbers of light-emissionpulses as luminance weights.

FIG. 1 is a block diagram showing a construction of this image displayapparatus.

As shown in the figure, the image display apparatus is roughly made upof an inverse gamma correction circuit 2, an addition circuit 3, astatic image coding circuit 4, a moving image coding circuit 5, a motiondetection circuit 6, a selection circuit 7, a sub-field control circuit8, a display control circuit 9, an AC-type plasma display panel 10(hereinafter, “PDP 10”), a differential circuit 11, a group ofcoefficient circuits 12, and a group of delay circuits 13.

The inverse gamma correction circuit 2 is a circuit which performs suchexponential correction as to decrease a reproduced luminance level whena gray level indicated by an input image signal 1 is low. Which is tosay, the inverse gamma correction circuit 2 is constructed so as tooutput a 12-bit image signal by adding a 4-bit decimal to an 8-bit inputimage signal. Given that the input image signal 1 is generally premisedon the inverse gamma characteristic of a CRT, in the case of a PDP thatdigitally controls reproduced luminance by the number of light-emissionpulses, the relation between an input gray level and a reproducedluminance level assumes linearity, as a result of which the gray levelcannot be expressed properly. The inverse gamma correction circuit 2serves to overcome this problem.

The signal having passed through the addition circuit 3 is supplied tothe static image coding circuit 4 and the moving image coding circuit 5.The static image coding circuit 4 has a look-up table that associateseach gray level with a value to which the gray level is to be converted.The static image coding circuit 4 performs coding in accordance withthis table. FIG. 2 shows part of the look-up table, where the leftcolumn shows input image signal values and the right column shows signalvalues to which the input image signal values should be converted.

As illustrated, input image signal values are basically converted to thesame values as the original, but some values such as “4”, “9”, “14”, . .. (designated by the thick line boxes 41) are converted to values thatare close to but different with the original values (e.g. “4” convertedto “5”, “9” converted to “10”, “14” converted to “15”). The purposes ofthis structure are to express every input image signal value with acertain value in correspondence with the coding in the sub-field controlcircuit 8 (i.e. the coding that divides into sub-fields withpredetermined luminance weights), and to cause such jumps in transitionsbetween luminance levels that will hamper consecutive luminance leveltransitions.

Likewise, the moving image coding circuit 5 has a look-up table thatassociates each input gray level with a value to which the gray level isto be converted, and performs coding based on this table. FIG. 3 showspart of the look-up table, where the left column shows input imagesignal values and the right column shows signal values to which theinput image signal values should be converted.

As illustrated, input image signal values are basically converted to thesame values as the original, but some values such as “4”, “9”, “14”, . .. (designated by the thick line boxes 51) are converted to values whichare close to but different with the original values (e.g. “4” convertedto “5”, “9” converted to “10”, “14” converted to “15”), as with thelook-up table in FIG. 2. This is intended to express every input imagesignal value with a certain value in correspondence with the coding inthe sub-field control circuit 8, as well as to cause such jumps intransitions between luminance levels that will hamper consecutiveluminance level transitions. Moreover, the moving image coding circuit 5exercises unique coding unlike the static image coding circuit 4. In themoving image coding mode, predetermined input image signal values suchas “40”, “50”, “70”, “80”, . . . (designated by the shaded areas 52),despite being able to be expressed on the PDP using sums of sub-fieldluminance weights, are converted to neighboring values in order toensure the correlation between changes of input image signal values andchanges of light-emission patterns of the predetermined number ofsub-fields (e.g. “40” converted to “30”, “50” converted to “60”).

FIG. 4 is a block diagram showing a detailed construction of the motiondetection circuit 6.

In the figure, the motion detection circuit 6 includes two framememories 61A and 61B each for storing image signals of one framesupplied from the inverse gamma correction circuit 2, a differentialcircuit 62, and a motion detection signal generation circuit 63.

The differential circuit 62 reads image signals of a current frame andimage signals of an immediately preceding frame from the frame memories61A and 61B, compares them for corresponding pixels, and calculates adifference for each pixel. The difference is then supplied to the motiondetection signal generation circuit 63. If the difference exceeds areference value, the motion detection signal generation circuit 63judges the pixel as having a motion status, whereas if the differencedoes not exceed the reference value, the motion detection signalgeneration circuit 63 judges the pixel as having a static status. Themotion detection signal generation circuit 63 generates a motiondetection signal indicative of the judgement result, and outputs it tothe selection circuit 7.

The selection circuit 7 uses the supplied motion detection signal thatindicates whether the pixel has a static or motion status, as aselection signal. According to this selection signal, the selectioncircuit 7 selects either an image signal outputted from the static imagecoding circuit 4 or an image signal outputted from the moving imagecoding circuit 5. The selection circuit 7 then supplies the selectedimage signal to the sub-field control circuit 8 and the differentialcircuit 11.

FIG. 5 is a block diagram showing a construction of the sub-fieldcontrol circuit 8.

As shown in the figure, the sub-field control circuit 8 is mainly madeup of a sub-field conversion circuit 81, a write address control circuit82, and frame memories 83A and 83B.

The write address control circuit 82 generates an addressing signalwhich specifies a write address in the frame memories 83A and 83B, basedon a horizontal synchronous signal (Hsync) and a vertical synchronoussignal (Vsync) which have been separated from the image signal.

The sub-field conversion circuit 81 receives the image signal from theselection circuit 7. The sub-field conversion circuit 81 is a circuitthat converts an image signal of each pixel that corresponds to thecurrent frame into field information of, in this embodiment, 10 bitswhich each have a predetermined weight. More specifically, each imagesignal for one frame is divided into a predetermined number ofsub-fields, based on a look-up table that defines converted informationfor a gray level of an input image signal (i.e. an input image signalbefore passing through the static image coding circuit 4 or the movingimage coding circuit 5). This division for each pixel image signal iscarried out in sync with a pixel clock generated by a PLL circuit (notillustrated).

The field information mentioned above is a group of 1-bit sub-fieldinformation indicating which periods within one TV field period, i.e.sub-fields, should be illuminated. For such generated field informationcorresponding to each pixel a physical address is specified by theaddressing signal outputted from the write address control circuit 82,and the field information is written into the frame memories 83A and 83Bfor each line, pixel, field, and frame.

FIG. 6 shows the correspondence between input image signal values (graylevels) and information to which each input image signal value is to beconverted, in the sub-field conversion circuit 81.

The figure shows a table of the correspondence between input imagesignal values and sub-field combinations after conversion, which is usedfor converting each input image signal value into 10-bit fieldinformation of ON/OFF states of sub-fields SF1-SF10 that have varyingluminance weights of “1”, “2”, “5”, “10”, “20”, “33”, “48”, “66”, “87”,and “111” in order of time. In the table, the leftmost column showsinput image signal values, whereas the remaining columns show 10-bitfield information to which each input image signal value should beconverted. In the field information, “1” means the pixel is ON(illuminated) during that sub-field. Otherwise, the pixel is OFF (notilluminated) during that sub-field (the same applies hereafter).

For instance, when an input image signal is “40” (designated by thethick line box 84), the sub-field conversion circuit 81 converts theimage signal to 10-bit data “0000100110” that shows the combination ofthe sub-fields with the luminance weights “2”, “5”, and “33”, andoutputs the 10-bit data. The bits here are represented in such a waythat digits in bit representation correspond to sub-field numbers.

The structures of the frame memories 83A and 83B are shown in FIG. 7.The frame memory 83A is provided with a first memory area 83A1 forstoring field information equivalent to the first half (1 to L (240)lines) of one frame, and a second memory area 83A2 for storing fieldinformation equivalent to the first half (1 to L (240) lines) of anotherframe.

The frame memory 83B is provided with a first memory area 83B1 forstoring field information equivalent to the latter half (L+1 to 2L (480)lines) of one frame, and a second memory area 83B2 for storing fieldinformation equivalent to the latter half (L+1 to 2L (480) lines) ofanother frame.

The first memory area 83A1 (the first memory area 83B1) and the secondmemory area 83A2 (the second memory area 83B2) each have 10 sub-fieldmemories SFM1 to SFM10. With these structures, two frames are eachdivided into halves, and field information showing a combination ofsub-fields of 10 bits for each pixel of each of the half frames iswritten in the sub-field memories SFM1-SFM10 as information concerningthe ON/OFF states of the sub-fields. In this embodiment, semiconductormemories of 1-bit input and 1-bit output are used as the sub-fieldmemories SFM1-SFM10. Also, the frame memories 83A and 83B are two-portframe memories in which writing of field information and reading offield information can be carried out simultaneously.

Writing field information into the four memory areas 83A1, 83B1, 83A2,and 83B2 in the frame memories 83A and 83B is performed alternately insuch a manner that field information for the first half of a frame iswritten the first memory area 83A1, field information for the latterhalf of the frame is written in the first memory area 83B1, fieldinformation for the first half of the next frame is written in thesecond memory area 83A2, and then field information for the latter halfof the next frame is written in the second memory area 83B2. Thiswriting of field information into each of the memory areas 83A1, 83B1,83A2, and 83B2 is done by directing each bit of the 10-bit data, whichis outputted from the sub-field conversion circuit 81 in sync with thepixel clock, to a different one of the sub-field memories SFM1-SFM10.Here, it is predetermined as to which bits in the 10-bit data should bewritten into the respective sub-field memories SFM1-SFM10.

The display control circuit 9 is roughly made up of a display linecontrol circuit 91, address drivers 92A and 92B, and a line driver 93,as shown in FIG. 8.

The display line control unit 91 indicates, to the frame memories 83Aand 83B, which of the memory areas 83A1, 83B1, 83A2, and 83B2, whichline, and which sub-field should be read to the PDP 10. The display linecontrol unit 91 also indicates, to the PDP 10, which line should bescanned.

The operation of the display line control unit 91 is synchronized withthe operation of writing into the frame memories 83A and 83B in thesub-field control circuit 8, in units of frames. That is to say, thedisplay line control unit 91 does not designate reading from the memoryarea 83A2/83B2 (or 83A1/83B1) in which field information is beingwritten, but designates reading from the memory area 83A1/83B1 (or83A2/83B2) in which field information has already been written.

The address driver 92A converts 640 bits of sub-field informationcorresponding to one line's worth of pixels, which have been seriallyinputted bit by bit in accordance with the memory area designation, readline designation, and sub-field designation made by the display linecontrol unit 91, into address pulses. The address driver 92A thenoutputs the address pulses in parallel to an appropriate line in thefirst half of the screen. The address driver 92B converts 640 bits ofsub-field information into address pulses and outputs them to anappropriate line in the latter half of the screen, in the same way asthe address driver 92A.

The line driver 93 designates, through a scan voltage pulse, a line onwhich sub-field information should be written in the PDP 10.

With such a construction of the display control circuit 9, fieldinformation is read from the frame memories 83A and 83B to the PDP 10 inthe following way. To read field information of one frame which has beendivided and written in the frame memories 83A and 83B, datacorresponding to the first half frame and data corresponding to thelatter half frame are simultaneously read. Which is to say, sub-fieldinformation for each pixel is sequentially read from the sub-fieldmemories SFM1, SFM2, . . . , and SFM10, simultaneously in the memoryarea 83A1 and the memory area 83B1. More specifically, sub-fieldinformation for each pixel of the first line stored in the sub-fieldmemory SFM1 is read bit by bit, simultaneously from the memory areas83A1 and 83B1. After a line designation is made by the line driver 93, alatent image is formed (addressing is performed) on the first line ofeach of the first and latter halves of the screen. Following this,sub-field information for each pixel of the second line stored in thesub-field memory SFM1 is read bit by bit simultaneously from the memoryareas 83A1 and 83B1, and inputted in the address drivers 92A and 92B inthe same way as above. Then sub-field information equivalent to oneline's worth of pixels, i.e. 640 bits of sub-field information areparallelly outputted from each of the address drivers 92A and 92B to thePDP 10, and addressing is performed. Once such reading (writing) hasbeen completed for the last line of each of the first and latter halvesof the frame, discharge pulses equivalent to the luminance weight ofsub-field SF1 are applied by the address drivers 92A and 92B, as aresult of which pixels are illuminated all at once.

After this, sub-field information concerning the ON/OFF state ofsub-field SF2 is read for each line and addressing is performed, in thesame way as sub-field SF1. Once this operation has been repeated foreach of the remaining sub-fields SF3-SF10, the reading (writing) ofone-frame field information ends.

FIG. 9 illustrates the drive method of the PDP 10. In the figure, thehorizontal axis indicates time, while the vertical axis indicates thenumbers given to scan/discharge-sustain electrodes running across thePDP 10. Each part with a thick slanting line denotes a period duringwhich addressing is performed on pixels to be illuminated, whereas eachshaded part denotes a period during which the pixels are illuminated. Tobe more specific, for horizontal pixels in a scan/discharge-sustainelectrode on the first line of each half frame, addressing is performedby applying address pulses to selected address electrodes running in thevertical direction, at the start of sub-field SF1. When the addressingends for the scan/discharge-sustain electrode on the first line, thesame operation is repeated sequentially for the lines that follow. Onceaddressing has completed for the last scan/discharge-sustain electrodein each half frame, the discharge sustain period t1-t2 starts. Duringthis period, the number of discharge sustain pulses proportional to theluminance weight of sub-field SF1 are applied to discharge-sustainelectrodes, where only pixels which have been addressed are illuminated.By repeating such addressing and simultaneous illumination of pixels foreach sub-field SF1-SF10, a gray-scale display for one TV field period iscompleted.

Concurrently with this operation, field information for the first andlatter halves of the next frame is read from the other memory areas inthe aforedescribed way. In so doing, successive images are displayed.

The addition circuit 3, the differential circuit 11, the group ofcoefficient circuits 12, and the group of delay circuits 13 areexplained next.

The differential circuit 11 calculates a difference between the imagesignal outputted from the selection circuit 7 and the image signaloutputted from the addition circuit 3, and supplies the differentialsignal to each of the coefficient circuits 12.

The coefficient circuits 12 have the coefficients 7/16, 1/16, 5/16, and3/16, respectively.

The delay circuits 13 delay signals outputted from the coefficientcircuits 12. Specifically, the delay circuits 13 delay by one pixel(1D), one line (1H)+one pixel (1D), one line (1H), and one line (1H)−onepixel (1D), respectively.

The addition circuit 3 performs addition on the image signal outputtedfrom the inverse gamma correction circuit 2 and the signals outputtedfrom the group of delay circuits 13, and supplies the outcome to thestatic image coding circuit 4, the moving image coding circuit 5, andthe differential circuit 11.

The above addition circuit 3, differential circuit 11, group ofcoefficient circuits 12, and group of delay circuits 13 form a loopknown as “error diffusion loop” that distributes the difference betweena gray level which is originally intended and a gray level which isactually displayed, to neighboring pixels.

(Effects)

By luminance-weighting the sub-fields in the aforementioned way, aheretofore unattainable wide dynamic range is realized while maintainingthe same level of resolution in a low gray-level range as conventionalPDP-equipped image display apparatuses.

FIG. 10 shows the correlation between input image signal values andreproduced luminance levels.

As shown in FIGS. 10(a) and 10(b), when the input image signal is in thelow gray-level range, the reproduced luminance level changes smoothlyand gradually with the change in input gray level, in both static andmoving images. For example, when the input gray level changes “0”, “1”,“2”, “3”, “4”, “5”, to “6”, the reproduced luminance level changes “0”,“1”, “1”, “2”, “2”, “3”, to “3”.

Meanwhile, when the input gray level is high such as when light is to beemitted during all sub-fields as shown in FIG. 10(c), the maximumluminance level is “1+2+5+10+33+48+66+87+111=383”, which is 1.5 timeshigher than the maximum luminance level “255” which was conventionallycommon. This enables an image to be displayed in a wide dynamic range.

Such a wide dynamic range is possible for the following reason. If allluminance levels (gray levels) which can be selected in the sub-fieldcontrol circuit 8 are rearranged in ascending order of luminance level(gray level), it can be seen that the luminance level jumps by one ormore levels at certain points (e.g. when the input gray level is “4”,“9”, or “14”). This makes it possible to increase the ratio betweenminimum to maximum luminance reproducible on the same screen, incomparison with the conventional techniques.

Here, to cause the luminance level to jump, it is of particularimportance to luminance-weight the sub-fields appropriately. That is,the sub-fields should be weighted so that a predetermined luminanceweight (e.g. the luminance weight “2” of sub-field SF2) is smaller thanone-half of the next luminance weight in ascending order (e.g. theluminance weight “5” of sub-field SF3).

Put another way, when the sub-fields are arranged in ascending order ofluminance weight with the “i”th smallest luminance weight being denotedby W_(i), then the luminance weights need to be assigned so that “n”exists where W₁+W₁+W₂+ . . . +W_(n)<W_(n+1). In the foregoing example,n=2.

To further widen the dynamic range, it is necessary to make theluminance level jump by greater amounts. Which is to say, when thesub-fields are arranged in ascending order of luminance weight with the“j”th smallest luminance weight being denoted by W_(j), the luminanceweights need to be assigned so that “n” and at least two “i”s existwhere W_(i)+W₁+W₂+ . . . +W_(n)<W_(n+1). In so doing, a wider dynamicrange can be obtained.

In the case of moving images, only part of the gray levels used whendisplaying static images is employed, as noted earlier. As examples, theinput image signal values “40” and “50” are respectively converted intothe image signal values “30” and “60”, as indicated by the shaded areas52 in FIG. 3.

What if such specific conversions are not conducted. In that case, whenthe input image signal is “40”, the three sub-fields with the luminanceweights “2”, “5”, and “33” are switched to the ON state, whereas thesub-field with the luminance weight “20”, which is ON when the inputimage signal is “30”, is switched to the OFF state.

This disturbs the correlation between the input gray level and thelight-emission pattern, thereby increasing the likelihood of falsecontours in a moving image display.

However, in the image display apparatus of this embodiment, the inputimage signal value “40” is converted to the image signal value “30” inmoving image coding. As is clear from this example, the embodied imagedisplay apparatus is constructed so as to switch no sub-field from theON to the OFF state or switch only sub-fields with relatively smallluminance weights from the ON to the OFF state, when the input graylevel rises. Hence the image display apparatus can display a movingimage without significant false contours.

As described above, the static image coding circuit 4 and the movingimage coding circuit 5 perform such coding that particular input imagesignal values are converted to values different with the original graylevels. This may result in an improper image display, as there is asignificant difference between the intended gray level and the graylevel actually displayed on the PDP 10.

To solve this problem, the error diffusion loop made up of the additioncircuit 3, the differential circuit 11, the group of coefficientcircuits 12, and the group of delay circuits 13 is adopted to distributethe difference between the intended gray level and the actual gray levelto neighboring pixels.

As a result, the jumps in luminance level transitions are compensated,and an excellent gray-scale image display is accomplished.

It is to be noted that the number of sub-fields and the luminanceweights of the sub-fields in the above embodiment are only presented byway of example, and should not be limited to such. If it is possible toincrease the number of sub-fields, sub-fields having smaller luminanceweights may be added to improve the resolution in the low gray-levelrange, or sub-fields having larger luminance weights may be added toimprove the maximum luminance.

Second Embodiment

FIG. 11 is a block diagram showing a construction of an image displayapparatus according to the second embodiment of the invention.

As shown in the figure, this image display apparatus has theconstruction of the image display apparatus of the first embodiment, andadditionally includes a display gray level scaling factor settingcircuit 14. The difference with the first embodiment lies in that codingin each of the static image coding circuit 4, the moving image codingcircuit 5, and the sub-field control circuit 8 is changed in accordancewith a maximum gray level of input image signals in a current frame. Thefollowing explanation focuses on this difference. Here, for the sake ofsimplicity, it is assumed that each input image signal is within a rangeof approximately “22” to “110” in gray level.

The display gray level scaling factor setting circuit 14 calculates ascaling factor of a maximum gray level of the current one-frame image(one-TV-field image) with respect to a reference gray level (e.g. thegray level “22”) (the scaling factor, hereafter denoted by K,corresponds to “a ratio of the sum of luminance weights of allsub-fields in the current TV field period to a sum of luminance weightsof all sub-fields in the reference TV field period” described in theClaims). The display gray level scaling factor setting circuit 14 thensupplies the value K to the static image coding circuit 4, the movingimage coding circuit 5, and the sub-field control circuit 8.

The static image coding circuit 4, the moving image coding circuit 5,and the sub-field control circuit 8 perform predetermined coding basedon the value K.

The static image coding circuit 4 performs predetermined coding thatdiffers in each of the cases where K=1, K=2, K=3, K=4, and K=5. Here,except when K=1, the static image coding circuit 4 executes such codingthat will jump by one or more gray levels (luminance levels). Thiscoding is carried out with reference to a plurality of look-up tables(similar to that in FIG. 2) which show the correspondence between inputimage signal values and converted (coded) gray levels for the respectivevalues of K. When K=2, K=3, K=4, or K=5, changes in gray level(luminance level) are not consecutive but particular gray levels(luminance levels) are skipped, as can be seen in the leftmost columnsof FIGS. 14(b)-14(e).

The moving image coding circuit 5 performs predetermined coding thatdiffers in each of the cases where K=1, K=2, K=3, K=4, and K=5. Here,except when K=1, the moving image coding circuit 5 executes such codingthat will jump by one or more gray levels (luminance levels). Also, themoving image coding circuit 5 limits coding to specific gray levels(each image signal value marked with an asterisk on the left of each ofFIGS. 14(a)-14(e) is unused. Likewise, each image signal value markedwith an asterisk on the left of FIG. 26(a) is unused). Such coding iscarried out with reference to a plurality of look-up tables (similar tothat in FIG. 3) which show the correspondence between input image signalvalues and converted (coded) gray levels for the respective values of K.

The sub-field control circuit 8 converts an image signal correspondingto each pixel into field information of, in this embodiment, 5 bitshaving predetermined luminance weights, with reference to coding tables(look-up tables) associated respectively with the cases where K=1, K=2,K=3, K=4, and K=5.

Conventionally, when switching between different coding patterns in thesub-field control circuit 8 based on the value of K, luminance weightsin a reference coding pattern (in FIG. 12, a coding pattern in FIG.12(a) where sub-field luminance weights are “1, 2, 3, 6, 10” in order oftime) are each multiplied by K to set luminance weights of a codingpattern corresponding to K, and each pixel within the current frame isdisplayed using the set coding pattern, as shown in FIGS. 12(a) to12(e). However, though this method can increase a maximum luminancelevel, it cannot widen a dynamic range of reproduced luminance levels.As is apparent from FIGS. 13(a)-13(e) concerning the correlation betweeninput image signal values and reproduced luminance levels, thereproduced luminance becomes higher as K increases, when the input imagesignal is in the low gray-level range (designated by the circles 201).This causes a decrease in resolution in the low gray-level range andmakes it impossible to widen the dynamic range. Note here that thedrawings on the right side of FIG. 13 are magnified views of therespective left-side drawings, where the corresponding drawings show thesame contents (the same applies to FIG. 15).

In the image display apparatus of the present embodiment, on the otherhand, luminance weights in a reference coding pattern (in FIG. 14, acoding pattern in FIG. 14(a) where sub-field luminance weights are “1,2, 3, 6, 10” in order of time) are multiplied by different coefficients.That is, smaller luminance weights are respectively multiplied by valuesno greater than K, whereas larger luminance weights are respectivelymultiplied by values greater than K, to set luminance weights of acoding pattern corresponding to K. Each pixel within the current frameis displayed using the set coding pattern.

Here, the coefficients for multiplying the luminance weights of thereference coding pattern may be coefficients that monotonously increasein ascending order of luminance weight.

Alternatively, the coefficients for multiplying the luminance weights ofthe reference coding pattern may be coefficients that increase inarithmetic progression in ascending order of luminance weight.

Alternatively, the coefficients for multiplying the luminance weights ofthe reference coding pattern may be coefficients that increase ingeometric progression in ascending order of luminance weight.

Among these, the use of geometrically increasing coefficients isparticularly effective for a wider dynamic range.

For instance, the coefficients for multiplying the luminance weights “1,2, 3, 6, 10” are:

“1, 1.5, 2, 1.83, 2.3” when K=2;

“1, 2, 2.67, 2.83, 3.6” when K=3;

“1, 2.5, 4, 3.83, 4.7” when K=4; and

“2, 3.5, 4.67, 4.83, 5.8” when K=5.

Here, when K=2 or K=3, a group of sub-fields whose luminance weights aremultiplied by values no greater than K includes a sub-field with aluminance weight multiplied by the smallest possible value of K (i.e.the coefficient “1”). In doing so, the increase of the luminance levelagainst the input is suppressed in the low gray-level range. Meanwhile,the larger the value of K, the larger coefficients are generally used tomultiply the luminance weights of the reference coding pattern, in orderto increase the maximum luminance level.

Thus, an image is displayed through the use of a coding pattern that iscomposed of a group of sub-fields having luminance weights multiplied bycoefficients no greater than K and a group of sub-fields havingluminance weights multiplied by coefficients greater than K.

By setting the luminance weights in such a manner, not only can themaximum luminance be increased, but the dynamic range of reproducedluminance levels can be widened. This is clear from FIGS. 15(a)-15(e)that show the correlation between input image signal values andreproduced luminance levels. The luminance level is kept low against theinput in the low gray-level range (shown by the circles 202), with itbeing possible to maintain the resolution in the low gray-level rangeand at the same time widen the dynamic range.

To further widen the dynamic range as the value of K rises, it isnecessary to make the luminance level jump by greater amounts. To do so,the ratios between the coefficients for multiplying larger luminanceweights and the coefficients for multiplying smaller luminance weightsare set to be greater for a larger K. Which is to say, when thesub-fields are arranged in ascending order of luminance weight with the“j”th smallest luminance weight being denoted by W_(j), a TV fieldperiod with a large K may be preferably given such luminance weightsthat “n” and at least two “i”s exist where W_(i)+W₁+W₂+ . . .+W_(n)<W_(n+1).

Take the aforementioned luminance weights when K=4 as an example. WhenW₁=1, W₂=5, W₃=12, W₄=23, and W₅=47, then n=4 and i=2 exist such thatW₂+W₁+W₂+ . . . +W₄(=46)<W₄₊₁(=47).

Thus, by making the luminance level jumps more sharply for a TV fieldperiod having a large K, the dynamic range can be effectively widened.

Note here that coding patterns are not limited to the above presentedpatterns. As long as each of at least two coding patterns includes atleast two approximate luminance weight proportions out of “1:2:3”,“1:2:4”, “1:2:5”, “1:2:6”, “1:3:7”, “1:4:9”, “2:6:12”, and “2:6:16”, itis possible to make the luminance level jump, as a result of which thedynamic range is widened.

Moreover, by using the error diffusion loop of the first embodiment todistribute the difference between the intended gray level and the actualgray level to neighboring pixels, the jumps in luminance leveltransitions are compensated, and an excellent gray-scale image displayis accomplished.

Third Embodiment

FIG. 16 is a block diagram showing a construction of an image displayapparatus according to the third embodiment of the invention.

In the figure, the image display apparatus has the construction of theimage display apparatus of the first embodiment, and additionallyincludes a space modulation circuit 15 for performing space modulationon a motion detection signal outputted from the motion detection circuit6, and a random number generation circuit 16 for supplying a randomnumber to the space modulation circuit 15. The following explanationfocuses on the differences with the first embodiment.

FIG. 17 shows, by way of example, an input image and a motion detectionresult in this embodiment.

When a triangular object 203 in FIG. 17(a) moves rightward as shown inFIG. 17(b), a solidly shaded area 204 in FIG. 17(c) is detected as amotion area which is represented by a motion detection signal, from thecurrent and past TV field periods.

The random number generation circuit 16 generates one of the randomnumbers such as “−3” to “3”, and supplies the random number to the spacemodulation circuit 15. The space modulation circuit 15 shifts the pixelpositions of the motion area shown in FIG. 17(c) in a horizontal orvertical direction by the number of pixels corresponding to the randomnumber, and thereby obtains a signal representative of a solidly shadedarea 205 in FIG. 17(d). The space modulation circuit 15 supplies theobtained signal to the selection circuit 7 as a switching signal.

Conventionally, coding is changed between a static area and a motionarea using the motion detection signal shown in FIG. 17(c) as aswitching signal. However, if the shape of the boundary of the motionarea represented by the switching signal is linear, the light emissionpattern accompanying the switching tends to get linear, thereby inducinga significant impact on the boundary of the motion area.

On the other hand, when the signal shown in FIG. 17(d) is used as aswitching signal, the boundary of the motion area represented by theswitching signal assumes a random shape. Accordingly, if such a signalis used to switch between the static image coding mode and the movingimage coding mode, these different coding modes will end up beinginterspersed in the boundary area. As a result, the switching betweenthe two coding modes will no longer cause a linear transition in timecharacteristic of light emission in the PDP 10. This makes the impact ofthe switching less significant, with it being possible to switch betweenstatic image coding and moving image coding more smoothly.

The above effects can be attained as long as the shape of the boundaryof the motion area represented by the switching signal is not linear.Therefore, though the pixel positions are shifted randomly in the aboveembodiment, they may instead be shifted regularly. Also, the sameeffects can be achieved when the boundary of the motion area has a shapethat contains a zigzag as a main component which turns no more than oncein a pixel.

Fourth Embodiment

FIG. 18 is a block diagram showing a construction of an image displayapparatus according to the fourth embodiment of the invention.

In the figure, the image display apparatus has the construction of theimage display apparatus of the first embodiment, and additionallyincludes a signal modulation circuit 17 for performing amplitudemodulation on a motion detection signal outputted from the motiondetection circuit 6, and a boundary detection circuit 18 for supplying asignal representative of a boundary of motion and static areas, to thesignal modulation circuit 17. The following explanation focuses on thedifferences with the first embodiment.

FIG. 19 shows, by way of example, an input image and a motion detectionresult in this embodiment.

When a triangular object 206 in FIG. 19(a) moves rightward as shown inFIG. 19(b), a solidly shaded area 207 in FIG. 19(c) is detected as amotion area which is represented by a motion detection signal, from thecurrent and past TV field periods.

The boundary detection circuit 18 detects the boundary of the motionarea where the value of the motion detection signal changes. Based onthe signal representative of this boundary area, the signal modulationcircuit 17 performs amplitude modulation on the motion detection signalin the boundary of the motion area, and thereby obtains a switchingsignal that represents a solidly shaded area 208 having edges 208A inFIG. 19(d). The signal modulation circuit 17 supplies the switchingsignal to the selection circuit 7. Note that in FIG. 19(d) the modulatedportion of the signal has a pattern where adjacent pixels alternatebetween two states.

When such a switching signal that has been modulated in the boundary ofthe motion and static areas is used, the boundary area assumes a randomshape as in the third embodiment, so that the different coding modes forstatic and moving images will end up being interspersed in the boundaryarea. Accordingly, if this signal is used to switch between static imagecoding and moving image coding, the switching will no longer cause alinear transition in time characteristic of light emission in the PDP10, as a result of which the impact of the switching becomes lesssignificant. Hence the switching between static image coding and movingimage coding can be done smoothly.

In addition, by modulating the motion detection signal in the boundaryof the motion area, the impact of the switching between moving imagecoding and static image coding is reduced, while coding modes for imageareas other than the boundary of the motion area are fixed. Henceunnecessary switching between the two coding modes is avoided, and animage can be displayed with no decrease in signal-noise ratio.

Though the modulation of the motion detection signal in the boundaryarea has a regular pattern in the above embodiment, the same effects canbe attained by modulating the motion detection signal using a randomnumber.

Fifth Embodiment

FIG. 20 is a block diagram showing a construction of an image displayapparatus according to the fifth embodiment of the invention.

This image display apparatus differs with that in the fourth embodimentin the following points. First, an addition circuit 19 and a randomnumber generation circuit 20 (which generates one of the random numbers“1”, “0”, and “−1” in this embodiment) are added to form a signalmodulation circuit. Second, three image coding circuits 21 to 23 areincluded instead of the static image coding circuit 4 and the movingimage coding circuit 5. Third, a selection circuit 24 having threesignal inputs is included instead of the selection circuit 7. Fourth,the motion detection circuit 6 detects movement in an image, byclassifying the amount of movement under three levels.

The image coding circuits 21-23 perform coding respectively under threelevels shown in FIGS. 21(a)-21(c). More specifically, for a static area,a coding mode shown in FIG. 21(a) is used that attaches importance togray-level characteristics. For a motion area, coding modes shown inFIGS. 21(b) and 21(c) are used that limit gray levels so as to suppressthe occurrence of moving image false contours. Of these, the coding modeof FIG. 21(b) is applied to image areas with intermediate amounts ofmovement, whereas the coding mode of FIG. 21(c) is applied to imageareas with relatively large amounts of movement.

The motion detection circuit 6 detects motion in an image under thethree levels corresponding to the three coding modes. The boundarydetection circuit 18 detects the boundary of the motion area where thevalue of the motion detection signal changes. The random numbergeneration circuit 20 generates a random number. The addition circuit 19adds the random number to the value of the motion detection signal inthe boundary area, and supplies the resulting signal to the selectioncircuit 24 as a switching signal.

With this construction, coding modes for image areas other than theboundary of the motion and static areas are fixed, so that unnecessaryswitching between static image coding and moving image coding is avoidedand an image display with no decrease in signal-noise ratio is produced.Also, intermediate coding is employed for an image area between thestatic area and the motion area in order to switch between the differentcoding modes step by step, which allows the switching to be madesmoothly. Furthermore, the switching signal is modulated in the boundaryof the motion area, so that the impact of the switching is effectivelysuppressed.

Sixth Embodiment

FIG. 22 is a block diagram showing a construction of an image displayapparatus according to the sixth embodiment of the invention.

As shown in the figure, this image display apparatus is equipped withthe boundary detection circuit 18 and the random number generationcircuit 20 (which generates one of the random numbers “0” and “1” inthis embodiment). Also, signal modulation circuits 25 and 26 forperforming amplitude modulation respectively on signals outputted fromthe static image coding circuit 4 and the moving image coding circuit 5are included in place of the signal modulation circuit 17 of the fourthembodiment.

FIG. 23 shows, by way of example, an input image and a motion detectionresult in this embodiment.

When a triangular object 209 in FIG. 23(a) moves rightward as shown inFIG. 23(b), a solidly shaded area 210 shown in FIG. 23(c) is detected asa motion area which is represented by a motion detection signal, fromthe current and past TV field periods.

The boundary detection circuit 18 detects the boundary of the motionarea where the value of the motion detection signal changes. The randomnumber generation circuit 20 generates a random number, and supplies itto the signal modulation circuits 25 and 26 as an operation switchsignal.

The signal modulation circuits 25 and 26 perform amplitude modulationrespectively on the image signals outputted from the static image codingcircuit 4 and the moving image coding circuit 5. The selection circuit 7selects one of the image signals outputted from the signal modulationcircuits 25 and 26, using the motion detection signal as a switchingsignal. As a result, an image signal shown by a solidly shaded area 211in FIG. 23(d) is obtained. Here, the modulated portion of the obtainedimage signal has a pattern in which adjacent pixels alternate betweentwo states.

When such an image signal which has been modulated in the boundarybetween the motion and static areas is used, that boundary assumes arandom shape, so that the static image coding mode and the moving imagecoding mode will end up being interspersed in the boundary area. Sincethe switching between these different coding modes no longer induces alinear change in time characteristic of light emission in the PDP 10,the impact of the switching becomes less significant. Thus, theswitching between the two coding modes can be done smoothly.

Furthermore, since the image signal is modulated in the boundary of themotion area, the impact of the switching is reduced, while coding modesfor image areas other than the boundary of the motion area are fixed.Accordingly, unnecessary switching between the two coding modes issuppressed, and an image is displayed with no decrease in signal-noiseratio.

Seventh Embodiment

FIG. 24 is a block diagram showing a construction of an image displayapparatus according to the seventh embodiment of the invention.

As illustrated, this image display apparatus is equipped with spacemodulation circuits 27 and 28 for performing space modulationrespectively on image signals outputted from the static image codingcircuit 4 and the moving image coding circuit 5, instead of the spacemodulation circuit 15 in the third embodiment. The following explanationfocuses on the differences with the third embodiment.

FIG. 25 shows, by way of example, an input image and a motion detectionresult in this embodiment.

When a triangular object 212 in FIG. 25(a) moves rightward as shown inFIG. 25(b), a solidly shaded area 213 in FIG. 25(c) is detected as amotion area which is represented by a motion detection signal, from thecurrent and past TV field periods.

The random number generation circuit 16 generates one of the randomnumbers such as “−3” to “3”, and supplies the random number to the spacemodulation circuits 27 and 28. The space modulation circuits 27 and 28respectively shift the pixel positions of the image signals outputtedfrom the static image coding circuit 4 and the moving image codingcircuit 5 in a horizontal or vertical direction, by the number of pixelscorresponding to the random number. The selection circuit 7 selects oneof these space-modulated signals using the motion detection signal fromthe motion detection circuit 6 as a switching signal. As a result, animage signal corresponding to a solidly shaded area 214 in FIG. 25(d) isobtained.

By using such an image signal that is modulated in the boundary of themotion area, that boundary assumes a random shape, so that the staticimage coding mode and the moving image coding mode will end up beinginterspersed in the boundary area. Therefore, the switching between thetwo coding modes no longer causes a linear change in time characteristicof light emission in the PDP 10. This makes the impact of the switchingless significant, so that the switching between static image coding andmoving image coding can be done smoothly.

Moreover, the image signal is modulated in the boundary of the motionarea, so that the impact of the switching is reduced while coding modesfor image areas other than the boundary of the motion area are fixed.Hence unnecessary switching between the two coding modes is suppressed,and an image is displayed with no decrease in signal-noise ratio.

In the third to seventh embodiments, a non-edge area that has littlechange in gray level is particularly susceptible to an impact ofswitching between different coding modes. Accordingly, it may be morepreferable if the procedure for preventing the linearity of theswitching is limited to the non-edge area. In so doing, not only can theimpact of the switching in the non-edge area be suppressed, but also theswitching of the different coding modes in an edge area can be performedswiftly. Hence coding that is suitable for each image area isimplemented without decreasing the average signal-to-noise ratio of thewhole image.

The combined use of the second embodiment and any of the third toseventh embodiments is possible.

In the second embodiment, the scaling factor K of the maximum gray levelof the current one-frame image (one-TV-field image) with respect to thereference gray level, which is calculated by the display gray levelscaling factor setting circuit 14, is assumed to be an integer. However,the scaling factor K does not have to be an integer but may be adecimal. FIG. 26 shows the coding pattern and the correlation betweeninput image signal values and reproduced luminance levels when K=2.5.

As shown in FIG. 26(a), among luminance weights of a reference codingpattern (i.e. the coding pattern shown in FIG. 14(a) whose sub-fieldluminance weights are “1, 2, 3, 6, 10” in order of time), the smallerluminance weights are multiplied by values no greater than K whereas thelarger luminance weights are multiplied by values greater than K, to setthe luminance weights for the coding pattern corresponding to K=2.5.Each pixel within the current frame is then displayed using this codingpattern.

Specifically, the coefficients for multiplying the luminance weights “1,2, 3, 6, 10” are “1, 1.5, 2.33, 2.5, 2.9”.

By such setting the luminance weights, not only can the maximumluminance be increased, but also the dynamic range of reproducedluminance levels can be widened. As shown in FIGS. 26(b) and 26(c)concerning the correlation between input image signal values andreproduced luminance levels, the luminance level is kept low against theinput in the low gray-level range, with it being possible to widen thedynamic range while maintaining the resolution in the low gray-levelrange.

In the above embodiments, the static image coding circuit 4 and themoving image coding circuit 5, or the image coding circuits 21-23,generate coded image signals which can be expressed by combinations ofsub-fields in the sub-field control circuit 8, and the selection circuitselects one of the coded image signals and supplies it to the sub-fieldcontrol circuit 8. Alternatively, the selection signal may be suppliedto the sub-field control circuit 8 so that the sub-field control circuit8 can convert the selected image signal to field information.

INDUSTRIAL APPLICABILITY

The image display apparatus of the invention can be used to display ahigh-quality gray-scale image that exhibits an excellent picture qualityor a wide dynamic range.

1-32. (canceled)
 33. An image display apparatus, in which a current TVfield period is divided into a plurality of sub-fields that arerespectively given luminance weights and are arranged in order of time,and a gray-scale image for the current TV field period is displayed bycoding input pixel image signals using different coding modes that areswitched in accordance with an amount of movement from an image of apast TV field period to the image of the current TV field period,wherein a combination of sub-fields is selected for each pixel dependingon the amount of movement, and a light emission state is sustained ineach pixel during the selected sub-fields, characterized in that asignal used for switching between the different coding modes isarbitrarily space-modulated so that the different coding modes areinterspersedly applied to input pixel image signals that correspond toan image area where the switching between the different coding modes isneeded and that show a predetermined characteristic.
 34. An imagedisplay apparatus, in which a current TV field period is divided into aplurality of sub-fields that are respectively given luminance weightsand are arranged in order of time, and a gray-scale image for thecurrent TV field period is displayed by coding input pixel image signalsusing different coding modes that are switched in accordance with anamount of movement from an image of a past TV field period to the imageof the current TV field period, wherein a combination of sub-fields isselected for each pixel depending on the amount of movement, and a lightemission state is sustained in each pixel during the selectedsub-fields, characterized in that a signal used for switching betweenthe different coding modes is regularly space-modulated so that thedifferent coding modes are interspersedly applied to input pixel imagesignals that correspond to an image area where the switching between thedifferent coding modes is needed and that show a predeterminedcharacteristic.
 35. An image display apparatus, in which a current TVfield period is divided into a plurality of sub-fields that arerespectively given luminance weights and are arranged in order of time,and a gray-scale image for the current TV field period is displayed bycoding input pixel image signals using different coding modes which areswitched in accordance with an amount of movement from an image of apast TV field period to the image of the current TV field period,wherein a combination of sub-fields is selected for each pixel dependingon the amount of movement, and a light emission state is sustained ineach pixel during the selected sub-fields, characterized in that asignal used for switching between the different coding modes, whenexpressed pixel by pixel as a virtual image of a matrix form in a plane,assumes a shape that contains a zigzag as a main component which turnsno more than once in a pixel, so that the different coding modes areinterspersedly applied to input pixel image signals that correspond toan image area where the switching between the different coding modes isneeded and that show a predetermined characteristic.
 36. The imagedisplay apparatus of claim 35, wherein the shape that contains thezigzag as the main component has a pattern in which adjacent pixelsalternate between two states.
 37. The image display apparatus of claim35, wherein the shape that contains the zigzag as the main component isa shape that randomly combines zigzags each of which turns no more thanonce in a pixel.
 38. The image display apparatus of claim 35, whereinthe input pixel image signals that show the predetermined characteristiccorrespond to a non-edge image area.
 39. An image display apparatus, inwhich a current TV field period is divided into a plurality ofsub-fields that are respectively given luminance weights and arearranged in order of time, and a gray-scale image for the current TVfield period is displayed by coding input pixel image signals usingdifferent coding modes which are switched in accordance with an amountof movement from an image of a past TV field period to the image of thecurrent TV field period, wherein a combination of sub-fields is selectedfor each pixel depending on the amount of movement, and a light emissionstate is sustained in each pixel during the selected sub-fields,characterized in that a modulation signal having periodicitycorresponding to no smaller than a pixel interval is applied to inputpixel image signals that correspond to an image area where switchingbetween the different coding modes is needed.
 40. An image displayapparatus, in which a current TV field period is divided into aplurality of sub-fields that are respectively given luminance weightsand are arranged in order of time, and a gray-scale image for thecurrent TV field period is displayed by coding input pixel image signalsusing different coding modes which are switched in accordance with anamount of movement from an image of a past TV field period to the imageof the current TV field period, wherein a combination of sub-fields isselected for each pixel depending on the amount of movement, and a lightemission state is sustained in each pixel during the selectedsub-fields, characterized in that input pixel image signalscorresponding to an image area where switching between the differentcoding modes is needed are modulated to shift a display position of theimage area.